tuklib_integer.m4: Optimize the check order.

The __builtin byteswapping is the preferred one so check for it first.

m4/tuklib_integer.m4

@@ -7,7 +7,7 @@
 #
 #   Checks for tuklib_integer.h:
 #     - Endianness
-#     - Does operating system provide byte swapping macros
+#     - Does the compiler or the operating system provide byte swapping macros
 #     - Does the hardware support fast unaligned access to 16-bit
 #       and 32-bit integers
 #
@@ -22,5 +22,20 @@
 AC_DEFUN_ONCE([TUKLIB_INTEGER], [
 AC_REQUIRE([TUKLIB_COMMON])
 AC_REQUIRE([AC_C_BIGENDIAN])
+
+AC_MSG_CHECKING([if __builtin_bswap16/32/64 are supported])
+AC_LINK_IFELSE([AC_LANG_PROGRAM([[]],
+			[[__builtin_bswap16(1);
+			__builtin_bswap32(1);
+			__builtin_bswap64(1);]])],
+[
+	AC_DEFINE([HAVE___BUILTIN_BSWAPXX], [1],
+		[Define to 1 if the GNU C extensions
+		__builtin_bswap16/32/64 are supported.])
+	AC_MSG_RESULT([yes])
+], [
+	AC_MSG_RESULT([no])
+
+	# Look for other byteswapping methods.
 	AC_CHECK_HEADERS([byteswap.h sys/endian.h sys/byteorder.h], [break])
 
@@ -25,6 +40,6 @@
 	AC_CHECK_HEADERS([byteswap.h sys/endian.h sys/byteorder.h], [break])
 
-# Even if we have byteswap.h, we may lack the specific macros/functions.
+	# Even if we have byteswap.h we may lack the specific macros/functions.
 	if test x$ac_cv_header_byteswap_h = xyes ; then
 		m4_foreach([FUNC], [bswap_16,bswap_32,bswap_64], [
 			AC_MSG_CHECKING([if FUNC is available])
@@ -44,19 +59,6 @@
 
 		])dnl
 	fi
-
-AC_MSG_CHECKING([if __builtin_bswap16/32/64 are supported])
-AC_LINK_IFELSE([AC_LANG_PROGRAM([[]],
-			[[__builtin_bswap16(1);
-			__builtin_bswap32(1);
-			__builtin_bswap64(1);]])],
-	[
-		AC_DEFINE([HAVE___BUILTIN_BSWAPXX], [1],
-			[Define to 1 if the GNU C extensions
-			__builtin_bswap16/32/64 are supported.])
-		AC_MSG_RESULT([yes])
-	], [
-		AC_MSG_RESULT([no])
 ])
 
 AC_MSG_CHECKING([if unaligned memory access should be used])